This invention relates to fluorescent lamps in general and more particularly to an improved solid-state fluorescent lamp ballast circuit.
Fluorescent lamps are gaseous discharge devices, and therefore typically exhibit a negative resistance characteristic during ionization. Ionization is due to the collision of electrons with the gas molecules contained within the lamp. The more current present in the arc, the lower the effective resistance of the lamp. Generally, a current limiting element must be introduced into the circuitry to prevent the lamp from ultimately destroying itself. The source of electrons for ionization is typically a cathode located in each end of the tube. Fluorescent lamps may be started without cathode preheat by increasing the applied lamp voltage until the free electrons are sufficiently accelerated to produce the energy required to ionize the gas. These voltage requirements are fairly high, dependent upon both the tube type and the operating frequency of the system. However, the starting voltage may be supplied from a reasonably low energy source, such as a high voltage pulse amplifier. Known current limiting and starting ballast functions usually consist of an inductive ballast designed for a particular gaseous discharge tube.
Most known inductive ballasts utilize some type of auto transformer configuration to provide the current limiting reactance and high voltage necessary to cause ionization. The light output, life and starting reliability of a particular fluorescent lamp depend upon the design of the ballast. Therefore, the size, weight and reliability of fluorescent lamp ballasts become very important factors in optimizing the total system efficiency.
Recently, there have been attempts to employ solid-state components for gaseous discharge to ballasts due to the possibility of achieving increased reliability with attendant decreases in the size and weight of such ballasts. Most solid-state ballast circuits utilize some form of semiconductor inverter circuitry to provide a separate frequency source, independent from the sixty cycle line frequency for driving the gaseous discharge to. Various types of semiconductor inverter circuits have been proposed, most of which utilized two or more power switching devices, normally power transistors, and one or more transformers to complete the DC to AC conversion function. One disadvantage of such an approach is the cost and size of the components necessary to provide the large amount of power utilized by known power transistors.
An example of an attempt to employ solid-state components in a ballast circuit for a gaseous discharge lamp may be seen in U.S. Pat. No. 3,896,336, issued to Max. P. Schreiner and Tom M. Hyltin.